1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus, a focus detecting apparatus, and an imaging system.
2. Description of the Related Art
An imaging system generally includes a focus detection (AF: Auto Focusing) sensor for detecting focus. Not only a large number of ranging points, but also highly-accurate, high-speed focus detection is demanded for the AF sensor of recent years. An area-type AF sensor is implemented as means for increasing the number of ranging points, in which a plurality of linear sensors forming the ranging points are arranged in parallel and connected by common wiring in a column direction. An example of a circuit configuration of the area-type AF sensor includes a solid-state imaging apparatus described in Japanese Patent Application Laid-Open No. H09-200614, the solid-state imaging apparatus including a transfer system for transferring a signal output from a sensor cell unit to a memory cell unit. A reset noise and a fixed pattern noise of the sensor need to be removed to increase the SN ratio of the sensor signal to improve the ranging accuracy of AF. In Japanese Patent Application Laid-Open No. H09-200614, the sensor cell unit and the memory cell unit have an inverting output function of signal to remove the fixed pattern noise of the sensor cell unit and the memory cell unit itself.
However, when the sensor cell unit and the memory cell unit have the inverting output function of signal as in Japanese Patent Application Laid-Open No. H09-200614, a tradeoff arises between the input/output linearity of signal and the circuit area. For example, a switched capacitor often used as an inverting output circuit includes a differential circuit and a feedback capacitor, and the circuit size is significantly large. The following three problems can be listed when a common-source inverting amplifier is used as in Japanese Patent Application Laid-Open No. H09-200614. Firstly, a semiconductor substrate bias effect degrades the linearity of an output signal. Secondly, the amplitude of an input signal changes the drive current of the inverting amplifier, and the consumption of current increases if the circuit responsiveness is improved. Thirdly, the relative variation of transistors varies the gains of inverting amplifiers, and the non-uniformity of output voltage is degraded.